One conventional power transmission structure is disclosed in JP-A-2004-359100 Publication.
This power transmission structure comprises a male yoke, a first sleeve, a second sleeve, and a lid member. The male yoke includes a shaft portion, and a connecting portion integrally formed at one end of this shaft portion. The shaft portion of the male yoke and the first sleeve are spline-fitted to each other so as to slide relative to each other in an axial direction. An outer peripheral surface of that axial end portion of the first sleeve remote from the connecting portion is fixed by welding to an inner peripheral surface of one axial end portion of the second sleeve.
The lid member is fitted in and fixed to an inner peripheral surface of the axial end portion of the first sleeve remote from the connecting portion. Grease is sealed in a grease sealing space formed by an axial end face of the shaft portion, the lid member and the first sleeve. In this manner, the grease is applied to sliding contact portions of the shaft portion and first sleeve over entire areas thereof, thereby preventing seizure from developing on the sliding contact portions of the shaft portion and first sleeve.
An air vent hole is formed axially through a central portion of the lid member. In this power transmission structure, the air vent hole is thus formed through the lid member. Therefore, when the shaft portion slides in the axial direction relative to the first sleeve to move toward the lid member, so that the air pressure within the grease sealing space increases, the air is discharged from this grease sealing space to the interior of the second sleeve. On the other hand, when the shaft portion slides in the axial direction relative to the first sleeve to move away from the lid member, so that the air pressure within the grease sealing space becomes negative, the air is introduced into the grease sealing space from the second sleeve. Thus, the air pressure within the grease sealing space is prevented from excessively varying so that the shaft portion can easily slide in the axial direction relative to the first sleeve.
In the above conventional power transmission structure, however, the distance between the above sliding contact portions and the air vent hole is small, and therefore the grease is liable to leak into the second sleeve through the air vent hole. When the grease thus flows into the second sleeve, an eccentric load acts on the second sleeve, which invites a problem that the power transmission structure is liable to be subjected to whirling. And besides, since the grease leaks into the second sleeve through the air vent hole, there is encountered a problem that the grease can not be sufficiently distributed to the sliding contact portions over the entire areas thereof for a long period of time.